Diffusion aluminide coatings are known to provide oxidation and corrosion resistance to many types of metal alloys. Diffusion aluminide coatings are particularly useful for improving the properties of parts fabricated from nickel and cobalt based superalloys used in high temperature environments such as gas turbine engines. Rotating blades and stationary vanes are examples of gas turbine engine components which are coated with diffusion aluminide coatings.
Diffusion aluminide coatings (also referred to simply as aluminide coatings) may be applied by two different processes, pack processes and gas phase processes. In a typical pack process, the part to be coated is embedded in a powder mixture within an enclosure or retort. The mixture contains a source of aluminum, a halide activator, and (optionally) an inert filler material which, in conjunction with the activator, controls the nature of the coating formed. To carry out the coating process, the part to be coated and the powder mixture it is embedded within are heated to a temperature of about 760.degree. C. in an inert atmosphere. At such temperature, the activator vaporizes and reacts with the aluminum source to form an aluminum rich halide vapor. (It should be pointed out that the coating industry describes the formation of gaseous activator as a vaporization process, as well as a sublimation or volatilization process. Such terms are used as synonyms herein.) The activator vapor reacts with the part to form a nickel aluminide or cobalt aluminide coating on the part surface, depending on whether the part is fabricated from a nickel or cobalt base superalloy. The thickness and composition of the aluminide coating depends upon the time and temperature of the process, as well as the activity of the powder mixture and composition of the part being coated.
Representative patents showing pack aluminide processes are those to Puyear et al., U.S. Pat. No. 3,079,276; Fitzer, U.S. Pat. No. 2,886,469; Supan, U.S. Pat. No. 3,335,028; Brill-Edwards, U.S. Pat. No. 3,693,255; Speirs et al., U.S. Pat. No. 3,764,373; and Boone et al., U.S. Pat. No. 3,544,348.
Some gas turbine engine blades and vanes are hollow, and have internal passages through which air is flowed during engine operation. The walls which define such passages require a coating to provide oxidation and corrosion resistance to the part. Pack processes are not efficiently utilized in coating internal blade and vane passages because of difficulties encountered in filling the passages with the pack powder mixture, and then removing such mixture after the coating process is completed. However, the gas phase diffusion aluminide process is particularly useful for coating internal passages. In the gas phase process, the part to be coated is suspended over a powder mixture within a retort which contains a source of aluminum, a halide activator, and (optionally) an inert filler material. The contents of the retort are heated to a temperature in the range of about 1,010.degree. C.-1,120.degree. C., which results in the formation of an aluminum rich halide vapor. The vapor is carried by a forced flow of inert gas into contact with the surfaces which define the internal passages and into contact with the external surfaces of the part, and the vapor reacts with the surfaces to form an aluminide coating To allow the carrier gas and the aluminum halide vapor to escape from the retort, the retort includes vents or other such devices.
Representative patents showing gas phase aluminide processes are those to Gauje, U.S. Pat. No. 3,486,927 and Benden et al., U.S. Pat. No. 4,148,275, the contents of which are incorporated herein by reference.
Diffusion coatings having compositions other than the above-described simple aluminide compositions are also known to the gas turbine engine industry. For example, U.S. Pat. No. 4,835,011 to Olson et al. describes an aluminide coating containing oxygen active elements like yttrium. U.S. Pat. Nos. 4,933,239 and 4,919,092 to Olson et al., and 4,897,315 to Gupta describe aluminide coatings containing nickel, cobalt, chromium, yttrium, and, optionally, silicon, halfnium and noble metals. U.S. Pat. No. 4,501,776 to Shankar describes aluminide coatings containing platinum. U.S. Pat. No. 4,148,936 to Grisik et al. describes a diffusion coating based on chromium.
One deficiency of state-of-the-art gas phase diffusion coating processes is that the halide activator sublimes at a temperature less than the temperature at which the coating actually forms on the part surface. (The temperature at which the coating actually forms on the part surface is referred to below as the coating temperature.) As a result, by the time the powder mixture and the part reach the coating temperature, much of the activator has sublimed and has passed out of the retort, through the vents in the retort, and is therefore wasted. For these reasons, the diffusion coating industry seeks a process for applying gas phase diffusion coatings more efficiently.